Harry Potter and the cryptographic key
by greatcactus
Summary: This is designed to start after chapter 102 of Eliezer Yudkowsky's "Harry Potter and the methods of rationality" (google hpmor). It was written by a fan waiting impatiently (but constructively) for the story to be finished. Read it before this. As usual, characters are owned by Rowling or Yudkowsky. Update: minor changes & note at end.
1. I feel a need for books

The restricted section of the Hogwarts library was, unsurprisingly, off limits to students. From under his cloak of invisibility Harry observed the fearsome barrier. A couple of poles and some half inch diameter rope suspended waist high.

It was fearsome, of course, by convention. Because it was prohibited. Because Madam Pince, the librarian, may notice if you stepped over the rope there without having provided a note signed by a professor. Because you might get a severe talking to.

Harry was an expert at being reprimanded. He would prefer to avoid it, all other things being equal, but at least he was sure that even in the madness that was Hogwarts, there would be no mortal danger from some tripwire. There was, of course, mortal danger in the books, but that was different. Books were Harry's friends.

The big question was whether there was more serious security than a rope and wandering librarian. In the muggle world, blueprints of nuclear bombs had more protection than a piece of rope in a school library. And there were potentially magics described over there far more destructive than a critical mass of uranium.

But then, security at Hogwarts was a joke. And the true cloak of invisibility would bypass most detection charms – at least those aimed at detecting people, rather than the movement of books. More prosaic detection systems wouldn't be fooled by the cloak. Harry had weighed himself while wearing the cloak. Gravity was not fooled - the scales showed the same result regardless of whether the cloak was hiding him.

To make a secure system, there are two approaches. For the correct approach, you need to clearly think about what you are trying to secure, and all possible attacks. The alternative, more popular approach, is to think of one possible attack, and defend carefully against that. That is a much simpler, more intuitive algorithm, and it leads to multiple strong locks on a door with hinges on the outside, or steel reinforced doors adjacent to thin glass windows. It leads to keeping Hermione at Hogwarts, for _safety_.

Harry noticed that his fists were clenched painfully. He consciously released them. Think clearly, Harry. You are going to do this. Nothing is going to stop you. Not Dumbledore, not whoever had killed Hermione, not even death itself. Nothing. Death was the enemy, and Harry refused to lose. Relax and think carefully.

_Evidence_: Dumbledore's previous behaviour. _Conclusion_: Dumbledore would have taken the popular approach. Therefore there was almost certainly a way to bypass whatever security the headmaster had set up for the restricted section, assuming he could work out what it was he had to bypass. But Harry didn't know enough magic to detect what charms Dumbledore had used. He would have to guess.

_Hypothesis: The security is aimed at people rather than books_. How could he test this, while maintaining plausible deniability? He could use minions, but that seemed unfair on them. Madam Pince was nowhere near. Harry took off his cloak of invisibility. Sure, he wore it routinely as a general security measure, but the use of it did unfairly prejudice observers if he had to protest his innocence. He waited until Madam Pince walked past, carrying a couple of books to reshelve. He waved his wand at a book he didn't recognise about two metres away in the unrestricted section of the library. _Accio Book_, he cast as quietly, as if avoiding disturbing other library patrons. Not that there were any at this hour. The book wafted gently over to him. Harry caught it – _99 popular uses of herbs_ - in his hands, and pretended to read it until Madam Pince was out of sight. Then he turned towards the restricted section, chose a book in his mind, and cast _Accio Book_ again, bracing for an alarm.

The alarm did not come. Neither did the book. Harry sighed, and replaced _99 popular uses of herbs_ in its proper spot, and put his invisibility cloak back on. That was his least desired outcome. If the book had come to him, with no alarms, he would have succeeded. If the book had come to him with alarms, then he could claim he had just mis-targeted his charm, and he would know more about the security system. But now all he knew was that at least one charm was blocked somehow.

There weren't many other options for him now. He could ask Professor Quirrell for a blanket permission slip. But even if the defence professor cooperated, it was not clear that this would be a long term solution. The restricted section was big. It would take him a long time to read enough of them. There would be plenty of time for Professor McGonagall or Dumbledore to notice and ask questions, and put a stop to it. He could ask Professor Quirrell for specific books – or books on certain subjects. That request would probably be successful – the defence professor had promised to help, after all. But the help may be censored, and Harry would have no way of telling.

None of the books would have a resurrection spell in them. Harry was pretty sure about that. Such a spell would hardly be obscure, even if it was impractical for most situations. Actually, it was surprising that there weren't even any rumours of resurrection spells, even if they were no more credible than the philosopher's stone ought to be.

Harry had identified four approaches to resurrecting Hermione.

Make up a resurrection spell.

Make up a "become god" spell, then use godlike power to make up a resurrection spell.

Find the Philosopher's stone, should it actually exist, and work out how to use it.

Other.

All four required more information, which probably wasn't even in the restricted stacks, but trying them couldn't hurt. Well, that was not exactly true. Sometimes a fresh way of looking at things was helpful, whereas reading what other people did would just give him a cognitive bias towards the unsuccessful class of approaches described in them. Harry's mouth twitched upwards at one side. Not looking at things the same way everyone else does was one of his core competencies.

Time was passing, Hermione's brain would be very slowly losing coherence, and someone was targeting his friends. It was time to try a riskier approach. Harry picked five particularly boring books from the unrestricted section, and then checked that the cloak was covering him. It was. He poked a foot under the rope barrier. "I was just sitting down here to read this book, and moved to be comfortable and accidentally stuck my leg under the rope" was somewhat plausible, if not convincing.

No alarm seemed to be going off. Harry waited there carefully for five minutes. Madam Pince did not come around.

Harry put the cloak back on, rolled his whole body through, and padded over to the bookcases. He skimmed over the titles – too many, too many – and chose five that may conceivably have something to do with spell creation. He replaced them with books of roughly the same size from the collection he had brought over from the unrestricted section. Lastly he padded back to the rope, under it, and back to the unrestricted side.

The piercing sound of alarms did not fill the library. That was worryingly easy.

Harry sat down to browse the first of his liberated books, "Magical Catastrophes"

* * *

><p>Several minutes later, Harry had started on the fourth book, "Dark rituals for fun and profit". All three previous ones had passed the "worth studying in detail" test, although it was hard to tell for sure.<p>

He jumped when he heard Professor McGonagall's voice a couple of metres behind him. "Mr. Potter. Please show me what you are reading. Now."

Still under the cloak, Harry quickly stuffed "Dark rituals for fun and profit" back into his pouch with the others, and pulled out "Magical Catastrophes". That would be easier to explain. Then he took off his cloak and turned around towards her. He held out the forbidden book. She glanced at the title, and coldly said "Please come back with me to my office."

They walked in uncomfortable silence, Professor McGonagall holding the book. When they eventually arrived in her office, she indicated at the guest chair with her wand, and sat in her own. She fixed him with a sad stare that hurt Harry more than any reprimand could.

"The centaurs believe that I am going to unintentionally cause a catastrophe," he said before she could start the conversation elsewhere.

"And you decided to do it intentionally instead? Or were you just curious about how you would do it?"

"No!" Harry paused "Well, yes, of course I was curious, but that is entirely incidental. I want to know how not to cause a catastrophe. For that I need to know what not to do."

"Most people manage to get through their whole lives without causing a catastrophe. Most of them would even say that 'don't do things that are forbidden as they are dangerous' is a good first step to not causing a catastrophe."

"But most people do not already have the centaurs prophesising that they will cause a catastrophe. Professor Quirrell is independently worried. For me to avoid disaster, I need to actively do something different."

"Don't you think that obeying the carefully considered rules would count as different for you?"

"That's… not a bad point." Harry shrunk back into his seat. _They may be carefully considered, but that doesn't mean they are sensible. But it also doesn't mean they are not. Why do I never think of the obvious, non-Harry-plan-centric view?_ Then he realised, and sat back up. "Sorry, forget that. It is a bad point. That is the sort of thinking that got Hermione killed." Professor McGonagall flinched. "I _need_ to be able to defend my friends."

"More than you _need_ to prevent a catastrophe?"

"Yes, I know the road to hell is paved in good intentions, but… "

"But you are different?"

"No! Yes! I am a scientist. I am rational. I understand that a small chance of disaster is not negligible."

"You don't make mistakes?"

This time it was Harry who flinched. "The more information I have, the better decisions I can make."

"I don't think that is true, Harry. If you don't know how to destroy the world, then you generally can't effectually decide to do so. If you have the information on how to destroy the world, then you can make a really bad decision."

"But I already know how to destroy the world! That's easy!" Harry realized how bad that sounded. "I mean, not as such, but I am sure I could come up with a couple of avenues for exploration given some thought. Destruction is easy. I could create a paradox with my time turner somehow, for instance." Harry kicked himself when he saw Professor McGonagall staring pensively at his chest where his time turner lay. "Not that I would, of course. Remember how terrified I was of it when you first gave it to me. There are lots of other ways. I could transfigure a huge pile of nuclear weapons. Or antimatter. Destruction is easy."

"You are not reassuring me."

"But I would have to be insane to do any of those things!"

"How many people in this school believe you to be sane, Harry?"

"But they are all wrong. Sure, many people think that I am mad because I behave rationally rather than following a script, but that is actually evidence of sanity. The people who really know me know that."

"The people who really know you are concerned about how Hermione's death has affected you."

"Oh." Harry deflated. In his heart he knew that the hypothesis _Harry has gone insane_ was not one he could categorically rule out.

"And if you knew some powerful, dangerous magic, can you say with absolute certainty that you would not have used it in the battle against the troll, under time pressure, with your best friend bleeding to death in front of you?"

Harry didn't answer.

"Harry, we give _transfiguration_ texts to first year students. The restricted books are _really dangerous_."

Silence stretched on for a while. "Thank you," said Harry at last. "Thank you for not just telling me off for being a naughty boy. Thank you for going off script and trying to give me useful advice and reasonable arguments. Thank you for treating me as an adult. That means so much to me."

"Does that mean I have convinced you?"

"No," said Harry slowly. "No, I cannot say that I am convinced. But neither am I convinced of my own position any more. I need to think about this for a while. Ummm. I have something to ask your opinion on. About my sanity. I want to see if you agree with Professor Quirrell."

"Go on."

"I want to resurrect Hermione."

"Impossible."

"Sorry, I phrased that incorrectly. I intend to resurrect Hermione."

"Intention doesn't make it possible."

"Sorry, I phrased that incorrectly. I will resurrect Hermione."

"Harry – I understand what it is like to lose a friend. But many, many wise and powerful magi have tried and failed. It is not possible. Believing it is possible is not sanity. Has Professor Quirrell been encouraging you?" Her voice was much sharper for the last question.

"No. He has been working hard to discourage me. He agrees with you that it is impossible, and I am mad to try. Actually, he thinks I am mad to want to do so, but that's just his issues showing. But not trying just because something is said to be impossible is wrong. Remember the partial transfiguration?"

"I do. Whenever I feel insufficiently terrified, I solve that problem by thinking of it."

"I've got so many things I _need_ to do," said Harry in a steadily rising voice. I _need_ to defend my friends against whoever is targeting them. I _need_ to defeat you-know-who, should he still actually be alive. To do that I probably _need_ to find out what power I have that the dark lord knows not. I _need_ to destroy the blight that is Azkhaban. And I desperately _need_ to not cause a catastrophe in the process. I _need_ to save the world from lots of problems it generally is unaware it has. I _need_ to not die before doing all these things. I _need_ to not make any mistakes." His voice suddenly became quiet. "But there is just one thing that I _want_. I _want_ to resurrect Hermione. Oh, and the defence professor too. Not that he is dead yet, but that should make fixing him easier."

Minerva looked at the first year with tenderness. So much we demand of him. So little we can offer him.

"You know if there is anything I could do to help, I would. But there isn't."

"But there is. I am going to get the power that the dark lord knows not. I am going to learn how to do the impossible – at least what he thinks is impossible. Everyone says that he is clever, so if he thinks something is inconceivable then probably most others do to. I have the power to conceive the inconceivable – yes, I understand that one could argue that is evidence that I am insane - but I need help. Concrete example: can I run any ideas that I come up with by you, as a sanity check?"

"Of course."

"And will you promise to take them seriously, and acknowledge the necessity of doing _something_?"

Professor McGonagall paused. It was not that the request was unreasonable, she just wanted to convince herself that she would actually be able to follow through with it. She finally replied, "Yes, as long as you promise to take my warnings seriously."

"Perfectly fair. I so promise. By the way, Professor Quirrell also promised to help me."

"What?"

"After trying hard to dissuade me from my resolve to resurrect Hermione, he offered to help."

"How?"

"By getting me information, among other things."

"So why didn't you get him to get you the restricted books?"

"I don't trust him. I don't trust him not to censor them."

Professor McGonagall paused, and said in a hesitating manner, "You have reason. I don't know if I should be saying this but… ten minutes ago it was he who told me you had gone into the restricted section of the library. He said it would be more effective for me to tell you off than him."

"What! Why is _he_ doing security for the restricted section?"

"He recently added some of his own security. He is afraid of you."

"_What_? Professor _Quirrell_ is afraid of _me_? He is absurdly powerful and smart and creative and devious and paranoid. He shouldn't be afraid of _anyone_."

"Harry, half the people who know you are afraid of you, and the other half are foolish. Or Griffindors." She paused, reflecting. "Foolish Griffindors. You successfully threatened some of the most powerful wizards in Britain at the Wizardgamot… and they had no idea of your true potential. Even dementors are scared of you. You calmly dismiss the threat of you know who. I, the head of Griffindor house, I am afraid of you, and I trust you to be well intentioned towards me."

Harry's face had grown steadily more horrified as she spoke. Minerva started speaking again. "Suppose you decided that you really wanted the defence professor dead. How long would he live?"

_Less than a day_, Harry thought as he considered the muggle materials he had got the Weasley twins to procure for him. "But I like him. I approve of life. I want to make the world a better place. I only want to destroy evil."

"Even so, would you ever think the ends justify the means?" It was obvious from Harry's expression what the answer was. She didn't wait for him to answer it. "But he is not, I think, worried about you actively going after him. He is worried about the mistakes you might make. Mistakes that humanity would not survive. Tell me Harry. Who scares you the most?"

Harry looked at the floor. He wanted to say the person targeting Hogwarts students. But that was not true. They were a minor threat. Once identified, he was pretty confident he could neutralize them somehow… probably by telling Dumbledore who it was. Or the defence professor if it were Dumbledore. Look at it impartially - what would the sorting hat say. That didn't need much thought. There was only one person he truly feared. "Myself," he said in a small voice.

"In your first few week at school you threatened myself, Professor Snape, and even the headmaster. They you started new research in transfiguration. Now you are searching the restricted section of the library. Am I wrong to be afraid of you?"

Harry suddenly straightened. She could see the self-confidence returning to him. "No, you are perfectly right to be afraid. So is Professor Quirrell. It is just that the idea of him being afraid is like the idea of Snape having a beloved pet bunny rabbit called Flopsy. I was distracted by the absurdity. You are correct to be afraid of me, but you should be even more afraid of a world without me."

"You mean because of you-know-who?"

"No, he is harmless compared to my real enemy. I mean death. It is not me you are scared of directly. It is not even what I might do. It is death you are scared of. Not for yourself, maybe, but for everyone. You are scared that I might cause everyone to die. Similarly, you are not scared of you know who. You are scared of the death and misery to many that he could bring. True?"

"Yes, but I don't think the distinction is anything other than pedantry."

"You fear death. That is the point. Consider that if I don't do something about it, you will die, as will everyone you know and care about. It may be an accident that gets them, or malice, or merely old age. But they will all die."

Harry paused for rhetorical impact, but started again before Minerva could interject. "I do not _fear_ death. I _hate_ death. I consider death to be an enemy to be vanquished. That is why the dementors fear me. I may not succeed, but if I fail others like me – and just as scary as me - will continue the fight. Eventually one of us will win, and humanity will look back on the era of death as a dystopia of fear and misery. You are right to be scared of me, but you should be even more scared of that which I will fight."

Professor McGonagall stared at Harry for almost a minute. Then she smiled. "If Gryffindor himself were here, I believe he would applaud. I am not him." Suddenly fire burned in her eyes. "But I have just decided to do something very Griffindor. I am about to do something ridiculously brave and stupid because my heart says it is the right thing to do. I am going to give you free access to the restricted section of the library, as long as you solemnly promise to run anything you come up with as a result by me. Do you so promise?"

"I do. I swear by my love for humanity to sanity check any ideas I get from the restricted section by you."

They talked for a while longer, about how to bypass Professor Quirrell's alarms. They settled on the obvious solution. Professor McGonagall would just get them for Harry, one bunch at a time. Harry requested ones related to disasters, healing, and the philosopher's stone. Minerva flinched slightly at the mention of the stone, but didn't say anything. Harry decided it was the wrong time to press.

Harry walked back to his dorm somewhat happily. Professor McGonagall was off script. The world was good. As long as he was sane, that is. If not… the world was in trouble.


	2. Lord Voldemort for a better tomorrow

Harry read. Mostly inside his trunk, wearing the cloak of invisibility on general principle, Harry read. He was _good_ at reading - almost as good as Hermione. But there was too much, far too much to read, and it was too hard to filter effectively. Furthermore it was hard to interpret many books without the context of other books that he hadn't read yet, let along the immense base of magic theory that adult wizards took for granted, but which a first year student at Hogwarts just had not yet covered.

Despite Professor McGonagall's astounding help in getting the books, Harry was not going to become a wise old wizard until he was, well, old. Hermione may not last that long. Hermione, who was genuinely good. It was so unfair that she should die, as the result of so much criminal incompetence. Incompetence of the magical healing profession at making defective first aid kits; incompetence of Dumbledore for not doing security properly; incompetence of himself for so many reasons; incompetence of the wizards of yore for not coming up with a decent resurrection spell; incompetence of the way that magic worked - it should make more sense.

There were so many reasons for anger, and one important use for anger. Cold clarity took over him. He turned back to the books, and read for a minute. Better, but not good enough. Not nearly good enough. This was not going to work. He needed a different idea. He needed to quickly learn how magic works. Then he could take revenge on all the incompetence.

Powerful magic was likely to be closer to the raw basic structure of magic. So it was still worth learning the contents of the books in the restricted section. But there were alternatives. Perhaps he should take Professor Quirrell up on his offer. Yes, right now.

Leaving his cloak of invisibility on, Harry wound his way to Professor Quirrell's office. Taking it off, he knocked on the door. There was no weak hesitation while in the grip of the cold clarity. The professor was possibly an enemy, could definitely not be trusted. Yet the probability of an attack today was low, and telling others about what he was about to do was not a good idea.

The door opened. Professor Quirrell was seated in a chair, reading a book that was propped a comfortable position in front of his face. Harry commenced without pleasantries. "You think I could perform a ritual powerful enough to destroy the Voyager probe. I wish you to teach me such magic."

"I do not know such powerful magic myself, and even if I did, it would hardly be wise for me to teach it to someone who conceivably may cast it himself."

"I don't intend to perform it, I just want to understand it. You promised to help me."

"And help you I will, to the extent of my ability, subject to general sanity limits. But I even if it were sane, I have no idea how to do so."

"Teach me the most powerful magic that I would be capable of understanding."

"Do you mind what it does?"

"Not really."

"Do you have to be able to cast it?"

"That would be helpful, but not essential."

"Do you realise that you are currently utterly indistinguishable from a dark lord? You even have the speech cadence down pat."

"I don't care. I am going to make the world a better place. I have been pussyfooting around, avoiding questionable means. But that time is done. The end justifies the means."

"All hail dark Lord Potter." Professor Quirrell bowed slightly, almost smirking. It didn't matter. He could have his fun. Harry waited coldly. "Let me make a prediction. I predict that in the reasonably near future, you, Harry Potter, will perform a powerful piece of black magic that most people would unquestionably consider the atrocity of the century."

"How will I learn this?"

"I will teach you."

"What will I do?"

"You will create Lord Voldemort."

"What!" The surprise was sufficient to break though the hold of the cold clarity, and normal Harry returned. "Why would I want to do that," he asked in genuine bafflement.

"Because the end justifies the means, and you want the end as it is good."

"What good end could that produce?"

"Let me start with a digression. Suppose you could change history. Suppose you could go back in time and stop Tom Riddle from becoming Lord Voldemort. Would you do it?"

"You can't do that."

"Most people would reply `of course'."

"But they don't understand the nature of time travel and paradoxes. That couldn't be part of a stable cycle. If Lord Voldemort had not been around, history would be different. I would not be special. I may not have my dark side - I definitely would not be as driven and as pampered. I would have no reason, and probably no ability, to go back in time and stop Tom Riddle. It wouldn't be stable."

"Possibly true."

"Of course, I don't understand time travel well enough to be entirely certain. As a best case result it is possible that a stable universe would somehow eventuate as the fixed point of many universes. But that would be very different to this universe, which would cease to exist, effectively killing everyone in it. That would clearly be worse for everyone currently alive than having Lord Voldemort around."

"I agree. So you wouldn't go back in time and stop Tom Riddle from becoming Lord Voldemort."

"No." As Harry said it, he made a mental note to go and think about this very carefully, as the chain of logic seemed sound, but the result was unintuitive enough to justify self doubt. Also, in the absence of Lord Voldemort, there would be no super smart Harry to overcome death for all humanity - a great - no, the greatest possible - end in itself.

"OK, let me talk about another hypothetical. Suppose you had the opportunity to do something that was massively evil, everything taken into account, including indirect ends. This is not a trick question. You wouldn't do it, would you?"

"No."

"Indeed, you would feel a strong moral obligation to not do so, correct."

"Correct."

"OK, now suppose you that the action and inaction situations were flipped. Suppose you could do nothing, and a great evil would come to pass. Or you could do something, and the evil would be stopped. Many people think that that is different; that one has a moral obligation to avoid evil actions, but one does not have nearly as strong an obligation to personally thwart evil. But you hold yourself to a higher standard; you would see the situations as morally equivalent, especially if you were the only one who could prevent the evil."

"Correct."

"You would feel a strong moral obligation to do the action that stopped the evil."

Harry could see where this was going, and he didn't like it.

"Correct."

"So, do you see what comes next?"

"There is some action that I could take - and only I could do so. That action would be retrospectively responsible for turning Tom Riddle into Lord V. And I must do it, because if I did not do so, then this universe we are currently in would not be a stable timeline, and thus would cease to exist, along with all the human consciousness developed since his accession."

"Indeed."

"But there are lots of problems with that, even ignoring the big picture of whether my model of the meaning of time loops is correct. Why me? Why now? Why does Tom Riddle need something from me?"

"Good questions. I am sure you can come up with answers for them."

Harry paused. "Why me? I guess I am special in two ways; firstly I am the boy who lived, and secondly I am smart, rational, and scientifically trained. The former seems more likely to be relevant given the nature of what is going on and the affinity I seem to have with Lord Voldemort. Oh. Oh."

Harry paused again, his mind racing with horrible thoughts. "It all makes sense. If I am responsible - at some point in the future - for creating you know who at some point in the past, then he could not have killed me back then, as that would cause a paradox. A time paradox trumps _everything_. The killing curse cannot be blocked, cannot be resisted by anything we know about. But a paradox is _impossible_. That has to be a fundamental law. The universe must contort itself to avoid a paradox."

That gave Harry a good idea. If he could make some situation where the only way to avoid paradox was for Harry's _Becomus Goddus_ spell to work... then the universe would rearrange itself, probably in such a way that humans never evolved, or something like that. No. He had done a close enough experiment already, and the conclusion was literally "Don't mess with time."

"Oh, and various other things could also make sense. Suppose I will have sent part of myself - say my dark side - back in time to fuse with Tom Riddle. That fragment would get stronger with practice, but could still have some affinity for me. When you-know-who tried to kill me, and instead burnt himself to a crisp, then the fragment could have partially jumped to me as it does with a Horcrux. Then, as I grew, it would grow itself again with practice. Looping around through time its strength would build up, and only stop – become stable - when the inefficiencies became big enough to counteract the experience gained over time. That would explain the power of Lord V - and of the dark side of my mind. It would also explain other affinities to Lord V, such as my ability to speak Parseltongue."

_Not to mention some other things that Professor Quirrell may be unaware of and which Harry was not going to give detail on. His wand being related to Lord V's. Above all, the prophecy. That would clearly explain _"_THE DARK LORD WILL MARK HIM AS HIS EQUAL" in a very literal manner. Furthermore, "EITHER MUST DESTROY ALL BUT A REMNANT OF THE OTHER" would also be literally true, almost axiomatic. And the power of paradox avoidance could be "HE WILL HAVE POWER THE DARK LORD KNOWS NOT." Assuming that Tom Riddle did not know where his fragment came from, then he would have no reason to suspect that it was part of a time loop._

"So," continued Harry after a pause. "So, it is plausible that I, Harry Potter the boy who lives, must do this to avoid the end of the world as I know it. Given that, why now? Ummm. That is not coming to me so easily. I guess it becomes harder as time goes on and the time span to be travelled increases, but that seems weak. Many years have passed; another is unlikely to make a huge difference."

"How will you know how to cast the spell," asked Professor Quirrell.

"Ummm. By the interdict of Merlin, someone would have to tell me. And who would be powerful enough to be able to do so, and would not baulk at the idea? Only you. And you..."

"And I don't have a very high life expectancy. You don't have to pussy foot around. I am not foolish."

Harry slumped. It all was plausible. He had even come up with all the reasoning himself. Ah! He had been expertly played. If the con man can persuade you to come up with the idea, then you will start off convinced. No, Harry would have to take this under advisement. But just because he had been played didn't mean it was incorrect. And if his failure to act would increase the number of pruned time loops, he had a moral requirement to take it seriously.

"What would be left of me," Harry asked at last.

"I don't know for sure. I am sceptical about the existence of genuine altruism, so I probably think there will be less left than you do. On the other hand, your personality is very different from that of you know who. So subtracting just his components should leave a significant part of your personality. You will probably lose some intelligence. I know how important that is to you."

"I need to think about this," he said.

"Of course," said the professor. "I needn't advise you not to discuss it with anyone. They will not treat it rationally. But before you go, I have a present for you. I think it will genuinely help you in your quest for a resurrection spell if anything can, and maybe make up for a potential loss of intelligence." Harry frowned. What could possibly make up for a loss of intelligence?

A drawer opened, and a sparkly piece of jewellery wafted towards Harry, who, after a moment's hesitation opened his hand and grasped it. He almost dropped it again; the feeling of doom associated with the professor seemed to pervade this item too.

"What is it? It looks like a tiara. I can't say it is exactly my style."

"Oh, but it is so exactly your style," said the professor in a thoroughly amused tone. "You are clearly the true heir of Ravenclaw. And it is her diadem."

Harry almost dropped it again. "I've heard of that," he said in a trembling voice. "It is said to raise the wearer's wisdom. If true, that would be, like, the most powerful magic item in the world. Magic is all very well and good, but intelligence is what separates us from the apes."

"It is indeed said to do so. And from my own experiments there is truth in the rumours. Although I think it raises intelligence rather than wisdom. I have made some inspired but ultimately unwise decisions while wearing it. Keep this in mind."

Harry stared at the artefact in his hands. The time turner was fantastic, but it had just been relegated to second favourite gadget. What could compare to a helmet of wisdom +5? He was just about to put it on, when he paused. Maybe he should get it checked for hostile enchantments first. That would be wise. Dumbledore would be ideal, but may not be trustworthy. Professor McGonagall it would be.

"Thank you! I mean. Wow! Umm. If you don't mind me asking. Where did you get it?"

"Did I steal it, you mean? Are you looking a gift horse in the mouth again? Don't bother to answer that. I found it in a hollow tree while I was in Albania. It appears the former owner, in her wisdom, managed to lose it.

"I should also warn you, Harry. I believe Lord Voldemort placed a Horcrux upon it. I don't believe it will affect you – I have removed the trap that would activate it, and it did not affect me. Besides, you are probably already infected, and thus have some level of immunity. But don't share it around – many of your friends are weak willed enough to be ensnared. It is unfortunately impractical to remove the Horcrux without destroying the diadem.

"You should see your face Harry. You are currently thinking that I couldn't possibly say anything that would shock you more than the conversation so far. So I can't resist even though I know it is rude. Thank you for getting the house points system suspended. That was a necessary step in my plot to get both Ravenclaw and Slytherin to win the house cup simultaneously. Murdering Hermione was the simplest method to accomplish that. Oh, look at your face. I got you. I am kidding about that. Of course I am. Ask Draco if you are not absolutely sure. Now scram, and use your diadem. Think wise thoughts. Everything else I said today I was serious about."

Harry put on his invisibility coat and scrammed. He had the best toy in the world to play with. But first he was going to check on something with Draco.

* * *

><p>Draco laughed at him. "Harry, Harry. You are so clever, and yet sometimes so spectacularly naïve. That would be a stupid plot. Even a play writer would be embarrassed by it. There are so many spectacularly bad potential downsides for such a plot, <em>and<em> the path to the desired outcome requires so much to fall the right way that no self-respecting Slytherin would consider it for a moment. It was a joke." Draco's smile vanished. "It was a joke in exceedingly bad taste."

Harry thought about power in a different way to most others. Harry's opinion of Draco would be enlarged by the truth in this case. In a small voice he would never reveal to anyone else, Draco added, "I miss her too." Then he put back on a wry smile. "Don't tell dad."


	3. My Precious

Before playing with his new toy, Harry wanted to check that is was safe.

He had briefly been concerned merely by the claimed function. This is a device that alters - ostensibly improves - how your _brain_ works. Harry was quite proud of his brain, and was rather attached to it. The potential side effects were terrifying, let alone personality change. But, frankly, that wasn't going to stop him using it. Not if it could help him save Hermione. Besides, he hoped that Rowena Ravenclaw was wise enough on the occupational health and safety front, although the artefact _did_ end up in a tree in Albania, if the defence professor could be trusted.

What he was really concerned about was the possibility of dark magic being overlaid. Well, clearly there was some dark magic already there, possibly a Lord Voldemort Horcrux, but that could be a blind. The defence professor had shown strong evidence of wanting to manipulate Harry down a particular path. He presumably couldn't use direct mind control magic on Harry for the same reason their magic didn't seem to work on each other. If the defence professor were out to control Harry, then a subverted mind controlling artefact that would be irresistible to Harry would be difficult to top as a perfect trap. And that was ignoring the possibility of a genuine Lord Voldemort Horcrux on it.

The feeling of doom that Harry got around the artefact didn't help either. Actually, it did. Suppose that the defence professor was correct about the Horcrux. Suppose further that the feeling of doom was associated with the Horcrux. Suppose that the reason the feelings of doom were similar was because the defence professor also had – or was – a Lord Voldemort Horcrux. Possibly Quirinus Quirrell had triggered a Horcrux, and been possessed by Lord Voldemort, or at least partly so. That was a lot of suppositions – a stiff complexity penalty should be applied.

What would the mixture of a maybe decent person and Lord Voldemort be? If the defence professor was correct about himself being part possessed by Lord Voldemort, then Harry was an example of the species. There were a lot of ways that could go. It certainly was not inconsistent with the defence professor's power, personality, and mysterious past.

Harry shook himself. Such speculation was important, but he had a shiny toy to play with. For all his misgivings, it took immense self control to not put it on _right now_.

Dumbledore would be the obvious person to ask about it, but Harry was somewhat leery of trusting him. Furthermore, Professor McGonagall deserved trust, and positive reinforcement. It was to her offices that he ran. After thirty seconds, he jumped on his broomstick. The less time he took to get to her office, the less time he had to resist putting it on.

Fortunately she was in his office, and called him in when he knocked.

"Harry! Don't tell me you have finished that set already?"

"No, sorry. I can't read that fast. Let along absorb and think about it. No, the defence professor just gave me the most powerful magical artefact I have ever heard of... and I want you to check it for booby traps."

She looked up in shock. "He gave you the elder wand? Not the Philospher's Stone?"

"No, no, way beyond them." He brought out the diadem. "_Rowena_'s artefact. Mind magic has the greatest potential for ... umm... greatness." Harry couldn't understand why everyone else didn't see this.

Professor McGonagall accepted the proffered piece of jewellery. It did indeed look like the pictures, and a cursory scan indicated that it was indeed endowed with significant magic. "This will take some time. It would probably be better to take to the headmaster, you know. He could do a better job than me."

"I trust him 95%. I trust you 100%. Please go ahead, I'll wait."

Minerva was shocked at the effect of his offhand declaration of trust towards her. She had lost much of her never particularly high trust in herself over Hermione's death. She had come to trust many of Harry's judgements - at least the ones that were not self-centred - and it appeared that he trusted her. Unlike Dumbledore, Harry wouldn't say it just to make her feel better. For the first time in her life, she thought that she might actually make a decent headmistress when and if her time came. She snorted to herself. It was pretty pathetic that such validation should come from a first year student.

She focussed on her work. First task was to determine if it were genuine. That was relatively easy. After only a minute and a half she looked up and informed him that she was quite sure it was the true diadem.

Searching for booby traps was more difficult. First a simple search for dark magic. Oh. "I have some bad news. There is some very powerful dark charm on the diadem. It is not part of the original enchantment. I will attempt to determine what it is." She spent another ten minutes, trying test after test. "The bad news is that I can't identify it for sure. It doesn't seem to _do_ anything. I suspect that it may be a Horcrux. We should really take this to the headmaster. He has more knowledge of Horcruxes than me."

"I thought you might say that. The defence professor warned me that he thought it had a you-know-who Horcrux on it. He claimed that it was unlikely to ensnare me. I didn't want to prejudice your examination by telling you in advance. Although if it looked as if you were about to put it on, I would have stopped you, of course."

"Despite this, you are still considering it? Can you imagine how terrible it would be if you were to become his host? Possibly that is what the prophecy meant by '_EITHER MUST DESTROY ALL BUT A REMNANT OF THE OTHER.'_ Could its power possibly be worth the risk?"

"I don't know. That is why I am asking you about it. It depends how big the risk it, versus the potential gain of giving me a significant advantage in a later confrontation with you-know-who."

Minerva considered it carefully. "This is beyond my area of expertise. I strongly recommend that we go to see the headmaster."

"I am happy to go to Professor Dumbledore then. Thank you."

"I haven't finished yet. I've checked for dark magic, but not for superficially harmless charms that could have less innocent uses. Give me a few more minutes." Before returning to the task she asked a witch in a painting behind her to see if the headmaster was in and free. She continued experimenting. By the time the witch had returned to her painting she had finished. There were no other charms.

The headmaster was indeed available, so they proceeded directly to his office. The door to the spiral staircase opened as they approached, the stairs twisting upwards.

Dumbledore sighed when he saw the two of them. "It is a bad sign when the two of you show up in my office," he greeted them. "Historically, at least. What has happened this time?"

Professor McGonagall replied, "Actually, it is quite good news." One of Dumbledore's eyebrows rose. "Harry was given Rowena's diadem by the defence professor. He asked me to check it for traps. I am convinced that it is genuine, but it has a strong black charm on it. I have suspicions as to its nature, but I would like your opinion too."

"May I see?"

Minerva handed the diadem to the headmaster. She watched him study it, noting many of the same spells she had used.

"It is a Horcrux. Almost certainly Voldemort's," he finally concluded. "This _is_ excellent news. We can destroy it, and that will be one step closer to his end."

"No," interjected Harry in alarm. "At least not until we have considered the alternatives. Unless - are you talking about just destroying the Horcrux, or the whole artefact?"

"The whole artefact, I am afraid, Harry. A Horcrux is too powerful to do otherwise."

"But Ravenclaw's diadem is the most useful item known to humanity!"

"Disposing of one of Voldemort's Horcruxes is of immense use to humanity."

"Could we hold off for a little, please? Does the Horcrux pose a direct threat to the wearer?"

"I do not believe it does. But Voldemort only has a small number of Horcruxes. The destruction of a single one would be valuable. Voldemort will only be genuinely dead when all are destroyed."

"It doesn't make sense," interjected Harry suddenly. "The diadem is a dumb place for a Horcrux."

"Why so?"

"Well, it is easy to notice and destroy, and hard to safeguard. More importantly, it is such a useful artefact that the possessor would want to wear it. A Horcrux, on the other hand, should be stored far away from its creator for redundancy purposes. It just doesn't make sense."

"On the contrary, it makes perfect sense," said the headmaster. "A Horcrux can be a trap. An unwary person might find a diadem, and put it on. This could trigger the Horcrux to take over their mind. So it makes sense to put it on a powerful artefact as it is a greater temptation to put it on."

Harry looked at his precious with some trepidation. "Is that an automatic property of a Horcrux, or does it need some other trigger?"

"It needs some other trigger. I couldn't find any traces of such a trigger."

"So it is a stupid place," said Harry.

"Maybe," suggested Minerva, "Maybe the other ones are in sensible places, and this one is in a stupid place to confuse us? Or maybe there was such a trigger, but the defence professor removed it."

Harry and Albus nodded weakly, acknowledging plausibility without being totally convinced.

Harry was meanwhile thinking about the Voyager spacecraft, and what an ideal Horcrux it would make, assuming some alien could trigger it. And Professor Quirrell had almost owned up to doing so, assuming of course that he was also a Voldemort mixture. "So there is little point destroying this one until we have found the others?"

"Destroying this one now would prevent us ever losing it," said Minerva pensively. "But there are alternatives. Albus, could you put a charm on it that would enable us to find it later if it were stolen?"

"Probably," allowed the headmaster dubiously.

"Headmaster, is your magic powerful enough to reach the planet Mars," asked Harry suddenly.

"No," said Albus, looking strangely at Harry. "Not even the moon."

"Do you think any other wizard has magic that could reach that far?"

"I would be exceedingly surprised."

"You know the muggles have had several spacecraft fly far beyond Mars. Ones that are never coming back. Ones that are unlikely to encounter anything as big as a speck of dust for the next billion years."

The headmaster and deputy stared at Harry in horror.

"I am sorry, Harry," said the headmaster finally. "If Voldemort is unkillable, and the prophecy says one of you and he are doomed to die, and the signs are showing that the battle is imminent..."

"Don't be defeatist," said Professor McGonagall briskly. "Firstly, if all of you know who is destroyed other than one Horcrux in deep space, then maybe that counts as `all but a remnant of'. In fact, that may fit the prophecy more than anything likely to happen to Harry. Furthermore, even if he keeps living, if his presence is with the spacecraft, it will be a long way away from anywhere he could do harm."

Harry didn't like that idea. It was an argument for the destruction of the diadem. "There are other interpretations. I could be left in a coma, in a vegetable state. That could be construed as `all but a remnant of.' Furthermore, I could always go and bring the spacecraft back."

The other two looked at him in astonishment. "How would you propose to do that," asked the headmaster.

"I don't know. But it is physically possible. I can't think of any particularly moral ways of doing it off hand - I could always imperious the muggle leaders of various countries and get them to mount a retrieval operation. But the cost would be stupendous. Even ignoring the unforgivable nature of the imperious charm, it would cost trillions of pounds - much more than the entire wealth of magical Britain. That money, spent on hospitals, would save the life of hundreds of thousands of people. That is not an acceptable solution. But maybe given time," _and the diadem_, "I can come up with a better solution. I'm just saying it is hard but not impossible. In the mean time, we should use whatever weapons we have, and my best weapon is my mind." He nodded towards the diadem. _And there is my precious_. I haven't even put it on, and still the power it may give me is irresistible, despite the clear danger.

The headmaster stared at him intently for several seconds. "Harry, can you imagine any reason why you would want to tear apart the very stars in heaven?"

"For energy," said Harry promptly. "Possibly for building materials. Most of the universe's accessible mass is probably in stars. Eventually humanity will want to use that mass more productively than just fusing it and radiating it indiscriminately. But that would be spread over billions of years at the earliest."

"How about ending the world?"

"That's a little harder. We could want to use the mass to build a Dyson sphere. That is, a huge sphere surrounding the sun with radius similar to the earth's orbit. It would have a surface area absurdly larger than the earth - lots of room for vast numbers of humans. It would make all the sun's output accessible. We will need that room and energy for the population explosion after I conquer death." He paused, and frowned. "But we would not have to make it exactly in earth's orbit, probably. A little outside, and we could adjust albedos so that the temperature was fine. Then we could use other planets, and keep the earth as a place to live on while we are making it. I would quite like to keep the earth afterwards anyway, as a souvenir of our ancestry. Maybe we would have to dismantle it to make the system stable. I don't know the maths of that offhand. Whatever, it is probably negligible compared to other structural problems involved in cosmic engineering, and vastly simpler than tearing the stars apart." _Even the Pak only built a ringworld_.

Minerva stared at him. It was the matter of fact way he said it that unnerved her, as if he were describing a change of room for some class. Albus was still watching him dubiously. _What made him ask_, she wondered.

"I think that Harry should be able to make the most use of his weapon," she said. "In fact, I think it appropriate than he should put the diadem on now, while we are here watching and capable of responding appropriately."

Harry needed no more urging. He held out his hand, the headmaster handed it to him somewhat reluctantly, he closed his eyes and put it on his head.

Nothing much seemed to happen. But Harry was short of input at the moment. He opened his eyes. At first everything seemed normal. He turned around and then he realised that he now remembered perfectly everything he had seen since putting the diadem on. Furthermore, as he gazed at the weird and wonderful contraptions around the headmaster's office, he guessed the functionality of two that had stumped him on prior visits.

"It works," said Harry, with a grin. The headmaster cast a few more charms on Harry - they seemed to be for sensing, and Harry could now remember in enough detail that he could probably reproduce them, although he had no idea how to interpret the results. Presumably the headmaster was checking that the dark charm had not activated.

"The dark charm seems quiet," said the headmaster. "You may not be able to see, but the diadem gave off a faint reddish glow when you put it on. I expect that is a result of its ordinary operation. Apparently when Rowena was wearing it there was a bright blue glow."

So it did not work quite as well for him as it did for the person whom it was designed for. Maybe it would improve with practice. Regardless, anything that improved his ability to think was priceless.

There was an obvious experiment to do. Harry took off the diadem. His flawless memory vanished as he did so. He still had some sense of what had passed while he was wearing the diadem, in fact it seemed a little better than he would normally remember. Harry smiled. "It is a wonderful device. Could you please add the locating charm, headmaster?"

He handed the diadem back to the headmaster, who waved his wand over it, then over a gem taken out of a drawer on the side of his desk. "I have made a bond to this gem. While I have the gem, I will be able to track the diadem. But please try not to lose it, Harry. We _will_ have to destroy it at some point." He handed the diadem back to Harry, who instantly put it on his head.

"Now," said Harry, "I need to think for a while."


	4. A little experiment

Harry put down the thirtieth restricted book. With the diadem, he could read much faster, and remember much more. But it still wasn't going to be enough. It still took time to think about the implications, to think about the assumptions that the author had made implicitly. To think about how magic works. The books were primarily about how to do things, rather than how to work out how to do things. This wasn't helpful when most of the things were not ones he had any interest in actually doing.

The fundamental problem, he thought, was that it just didn't make sense. Magic behaved like a story, not like a coherent basis for the fundamental properties of nature. Take the spell words. Despite their use of dead languages, they were really very twentieth century. They were an anachronism. They were what a well-educated kid designing them in a hurry would make up to sound plausible. But they had too much modern English implicit in them to be made up when magic was young. And given the interdict of Merlin, and his and Hermione's experiments in changing the words, they were not a modernization of older originals. They were the sorts of things that _Ron_ would make up.

Harry snorted. If he had designed magic, it would be done differently. Maybe not the words – they were not worth wasting time on. The edges of his mouth twitched up. If he had designed magic, he would have put a back door in. A backdoor that would allow him to take control of all the magic in the world, say by brandishing a key, and uttering the spell _Becomus Goddus_. And the key... the key would not be some exotic artefact, it would be...

Harry knew what it would be.

He paused. If it were possible for him to retrospectively create Lord Voldemort, then there were a couple of plausible reasons why he should be able to create the key.

It wouldn't take much time to check.

How long would a key have to be to be unique in all universes? Harry remembered Dirac talking about some relevant things. The diadem even improved his memory for things he had read before wearing it. The biggest dimensionless numbers associated with the product of the age and size of the universe were of the order of ten to the power of a hundred and twenty. That's about four hundred bits. Add another hundred to wipe out coincidences. But there are an infinite number of universes possible through time loops. Maybe it is not infinite. Assume it is similar to the size of the universe. Double the number of bits to a thousand. Double it again to make up for messy back of the hand calculations. Double it another two times for unknowables, and to make it a nice round cube. Eight thousand bits.

Harry looked at his father's rock, or at least the transfigured version of it on his ring. Harry caught himself. He shouldn't call the ring his. That was horrible. Disgusting even. An unintentional slip of the metaphorical tongue was worse than it appeared, as it could lead him to be careless and selfish. He shook himself and looked again at the gem that had distracted Dumbledore from the far more important ring. It would suffice. He removed the brown gem from the ring; it would never do to scratch the ring. Then Harry thought of a grid of eight thousand dots arranged in a cube twenty on a side. He transfigured the gem slightly, and after a minute's work a tiny version of the grid appeared inside the gem. It was a good start.

Incidentally, Harry noted that it would have taken him roughly five times as long to do such a fine transfiguration without the diadem.

Now for the boring part. "Randomization device," he said to his pouch. Out came a galleon. Harry flipped it. Heads. He flipped it again. Heads again. Then tails, heads, tails, tails and heads again. He repeated this until he had eight thousand flips… it took about two hours. With the diadem on, he could remember them all. He took the diadem off for a minute. Then put it back on. He could remember the flips again. Perfect.

Harry visualized the cubic grid again, except this time he mapped the series of eight thousand flips onto the grid, one flip for each point. The points corresponding to a heads flip, he omitted from the grid. He now had four thousand and thirty seven points in a random pattern. It was a key; a key that only he knew. Barring time travel sending its details into the past, it was a key that with should have never been seen elsewhere in the universe. He transfigured his father's rock to include this pattern.

The gem shone fiercely. Harry fell down backwards in surprise. The diadem fell off. Instinctively Harry put it back on his head and looked at the key in awe.

There were three obvious possibilities, all with a tiny prior probability. But given the experimental evidence that the key had done something strange, one of them was probably true.

First possibility. He, Harry James Potter-Evan-Verres, will have had created magic retrospectively, and tied a root access to this key.

Second possibility. The key was the key to someone else's creation, and the reason that Harry had determined the key was that the only stable time loop was one where the flips happened to work for Harry.

Third possibility. Someone was playing a very good practical joke on Harry. Probably his sadistic future self.

Some of the elation left him. The practical joke possibility almost certainly had the highest prior probability.

Time for test number two. Wishing he had a video camera to catch the possibly most important event in the history of the planet, Harry said "Becomus Goddus".

Nothing happened for a moment. Then the gem beeped like a microwave oven and said, in Harry's smuggest voice, "User too evil error."

"You bastard," he swore at himself. How did people ever put up with him?

Well, that was mild evidence against his second theory, and fairly strong support for the practical joke hypothesis. But if he had indeed created a backdoor into magic, he would have thought for a while about it, and maybe he would have put in a check for moral fitness. Forget the maybe, this was dangerous. Of course he would have done so. Sanity too.

So now what? Should he try to become a better person? Was that the whole point of the practical joke? Maybe Dumbledore set the whole thing up in order to make Harry a better person. Call that hypothesis four. Suddenly Harry felt very cold. Hypothesis five. Professor Quirrell had set it up to persuade Harry to go through with the ritual he was suggesting. That had to be at least as likely as a practical joke from his future self.

Time to eliminate hypothesis three, anyway. Time to visit Professor McGonagall again. The gem was too potentially dangerous to transport; he transfigured it back to a perfect gem. It stopped glowing.

* * *

><p>Minerva opened the door after hearing a knock. Harry appeared in the empty corridor a moment later, taking off his invisibility cloak. "Don't tell me you have an even more powerful artefact you wish me to examine for interference from some dark lord," she welcomed him.<p>

Harry blinked.

"Ummm. Not exactly. I used to have it, but I destroyed it. It was too dangerous to carry. But I can easily make it again."

"And what is it?"

"Well, I am not exactly sure. Possibly it is the key to the become god spell, or possibly it is a practical joke played on me by… ummm… someone brilliant and exceedingly annoying." Harry looked positively embarrassed at this point. "Or maybe it is something else."

"And how did you come to have possession of this artefact?"

Harry explained what he had done. His sentences made sense, and indeed it was a clear explanation of a set of actions. But it was missing an important prequel. "But what book told you to do this? And I thought you had promised to consult with me before doing any ritual from the restricted books."

"Oh, it wasn't from any of the books. It just seemed like an obvious thing to do. I mean if I had been the one who designed magic, this would have worked." He looked somewhat embarrassed at this.

Minerva was worried that something seriously wrong had happened to Harry's common sense. She wondered how to test it. The normal approach would be to ask the student some simple questions and see if the answers were normal. However, normal for Harry was hard to judge. Then she had an inspiration.

"Harry. If I wanted to check that the diadem had not damaged your brain, and that your common sense still was running, what would be a good approach?"

"I don't approve of common sense," he relied loftily. "I have _uncommon_ sense, and that is hard work. But seriously, let me demonstrate. It is a very simple transfiguration of a gem. If it works, then it demonstrates that I am at least a little sane. Sorry, that is not going to make you comfortable. The idea of using transfiguration as a test of sanity is like using a flaming brand to look inside a gunpowder barrel to see if it is empty."

That sounded normal for Harry.

"Actually, it would be a better test if I did it on something different. It should be the pattern rather than the material, and it makes it harder for someone else to interfere that way. Could you please, professor McGonagall, choose something other than a rock and transfigure it into a gem somewhat different to this one?"

There was a glass on her desk. She tapped it with her wand, and it became an irregularly shaped blood red translucent gemstone. Harry then pointed his wand at it. Thousands of tiny imperfections appeared in the centre, and then the gem began to glow, and give off an aura of power greater than Minerva had ever seen before. She gasped in surprise.

"I have so many questions about this," started Harry, "but one of the first is, how much power would it take to make this, if it were not some intrinsic property of the pattern but some externally imposed charm, done without us noticing?"

Minerva studied it carefully for a couple of minutes. There was no doubt it was powerful, absurdly powerful. But it didn't seem to _do_ anything. The enigmas in the headmaster's office at least had some clues to their function.

"It is truly powerful," she said at last. "I would be surprised if even Merlin would have the power to create it in front of us without us knowing. But then I am surprised at this thing's existence. Perhaps a mage with this could be more powerful than Merlin. You made it, for reasons I don't understand. What do _you_ think it is for?"

Harry looked embarrassed. "I thought I could use it to cast the _Becomus Goddus_ charm."

Suddenly the gem spoke with Harry's normal voice. "User too evil error." Harry cringed.

"And where did the _Becomus Goddus_ charm come from?"

"User too evil error."

"Interesting," said Harry. "It still used my voice and… questionable taste in diagnostic messages when you tried to become a god. That is evidence against it being set up thousands of years ago by a third party."

She was holding a device that could conceivably turn someone into a deity.

She just invoked it accidentally.

She was considered too evil!

Harry seemed utterly unconcerned by any of this.

Minerva couldn't determine which of those ideas was the most shocking.

"Don't be offended, please," said Harry. I don't think its meaning of evil is normal. Whoever set up the conditions where this device would be created by the key I used wants it only to work for someone who is morally worthy of being a deity. Would you trust me to be a god?"

"No. Nothing personal, I don't think I would trust anyone, including myself."

"Agreed. But then why bother having it in the first place? A cosmic joke to torment the arrogant? Or maybe there exists some perfectly noble soul?"

"Not that I know of."

"Or maybe there is some way to make a normal human morally better. Could you Imperius me and make me permanently _good_? Make me the way I would want to be if I were as noble as I would want to be if I were as noble as I would want to be if etc."

"I doubt it," she replied. "Besides, something sophisticated enough to deny godhood to someone based on their moral character should certainly deny it to someone in the thrall of someone of presumably worse moral character."

"Yes. I would probably have thought of that. So yes, the creator probably would have. I know! The sorting hat. It has seen lots of children's minds. It would know if it had ever seen anyone suitable. It might even tell us." Or at least tell you. It doesn't like me. Although I would like to see if it still thinks I may turn into a dark lord. Oh, no, I shouldn't have suggested this… she may see the implication of consciousness implied by such communication. "Actually that is a stupid idea I should not have suggested. Maybe we should ask Dumbledore?"

Minerva nodded.

* * *

><p>"You two are getting very chummy, I must say. I quite approve. What can a somewhat wise old wizard do for you today?"<p>

"I have another artefact I would like you to examine, please," said Harry as he brought the blood red gemstone out of his moleskin pouch with the keyword "transfigured glass."

Dumbledore had initially shown polite interest at the question, but when he saw what Harry had he dropped the quill he was holding. For at least half a second he appeared utterly shocked. "That is clearly a powerful artefact," he said a second later, far too casually. "May I investigate?"

Harry handed it over without a word. While Harry had some suspicions of Dumbledore, he was blithely unconcerned about anyone abusing this artefact. Anyone it allowed to ascend to deityhood would, by design, be an acceptable person to use it. Not necessarily the best person – that would be himself, of course. And of course he wasn't worried about losing it - if Harry needed it later, he could just make a new one.

Dumbledore stared at it intently for almost a minute before sighing and dropping it into a glass of water on his desk. He then stuck a finger into the water for three seconds, removed his finger and carefully studied its tip. Harry was too far away to see any detail on his finger – it certainly looked healthy as far as Harry could see. Next Dumbledore retrieved some tongs from a drawer in his desk, and used them to extract the gemstone which he carefully dried with a towel. He put the gem down on the table, and then drew his wand – Harry had read enough now to recognise it as the elder wand – and cast _finite incantatem_ at the glass of water. Having done so, he poured it carefully down the sink.

Professor McGonagall started to say something, but the headmaster put a finger to his lips in the universal shushing gesture. He waved his wand again, and a silvery phoenix appeared. "Go to Nicolas Flamel," he said. "Ask him to please come here as a matter of urgency."

Next the headmaster walked over to Fawkes' roost. The real phoenix jumped onto his shoulder. Dumbledore then grabbed the roost and broke it over his knee. A blood red gemstone fell out of the middle, twin to the one Harry had brought in.

Dumbledore then repeated the glass and water experiment with the gemstone from the roost, using a different glass and a different finger.

"Identical," said the headmaster finally. At that point the patronus phoenix returned, and reported "He says he cannot come, but says to tell Harry to do it." Dumbledore dismissed the patronus, and turned to Harry. "In most other circumstances I would want to know how Nicolas knows you are here. My office is quite secure against remote scrying. Also I must ask you about what you are considering doing. But there are more important questions first. How did you possibly manage to get a duplicate of the philosopher's stone?"

Harry almost choked. _So that's what the philosopher's stone is. And I didn't believe in it because immortality and gold production were absurd fanciful powers. They are presumably just minor side effects of the main purpose_. "I made it," he said simply. "Well, Professor McGonagall transfigured a glass into the gemstone, and then I transfigured a pattern of defects into it."

"It is true, Albus," she verified. "I observed him do so. But what was the original doing in Fawkes' roost? I thought it was protected in the underground chambers by the combined efforts of the faculty."

"So do many people. But the obstacles placed there were hardly inviolable. A sufficiently powerful or ingenious wizard could easily bypass them. It seemed sensible to give people an obvious and noisy target, when really it was hidden in my office which is, actually, very well secured. The odds of someone stumbling over it were low, and no one would be suspicious if they detected a large concentration of magic in a phoenix's roost."

Harry's opinion of Dumbledore's sanity increased. Hiding it in a flashy manner, requiring heroic endeavour to overcome but not actually making it truly secure was very much the vibe that the headmaster gave off, and that Harry had believed. Everyone knew that Dumbledore was not as insane as he pretended – that he was really no more insane than an average person not steeped in the art of rationality. But maybe he was significantly less insane still than that. Whatever, Harry was certainly going to have to update his model of Dumbledore. He suddenly realised that it was possible that Dumbledore had realised all along many of the things Harry had done… such as the Azkaban breakout. Or worse. Maybe he and the defence professor had planned it together. It would explain why Harry had had a suspiciously easy time proving his innocence, or why when the headmaster was searching for Hermione's body he didn't cast _finite incantatem_ on the ring, just the gem. Harry mentally shook himself. This should be thought about later.

"Anyway, you, Minerva, transfigured the glass into a gem with the exterior and colour of this. Correct?"

"Yes."

"Why did you choose that in particular?"

"No particular reason. I was trying to come up with something unlike the prior instance Harry had made. It just seemed like a reasonable shape. The colour I copied from the glow on the diadem, just on a whim."

"Have you seen the philosopher's stone before?"

"Never."

"Could someone have been inside your mind at the time?"

"I don't think so. I was being unusually vigilant, as Harry thought that the whole thing might have been some practical joke inflicted by some powerful third party. Although the coincidence seems improbable."

"Is time travel beyond the six hours of a time turner possible," asked Harry. He was somewhat concerned about the possible fallout for this discussion, but it would be worth it for information relevant to the defence professor's proposed ritual.

"There have been occasional rumours," said the headmaster. "But they are difficult to substantiate without causing paradoxes, which in turn would make them impossible. I know of no safe way to do so. Destroying a time turner sometimes may have that effect, although it is hardly safe."

"The hypothesis I am concerned about," said Harry, "is the third party mentioned by Professor McGonagall, in particular if they are malevolent. Suppose someone wanted to make me think that I could create the philosopher's stone. Maybe when I was doing some experiment, the deceiver would simultaneously perform a more powerful magic, actually making it."

"Harry," said the headmaster. "Even prior to your creation of the most powerful known magical artefact, I respected your abilities, and consider you a credible thread to Lord Voldemort. However anyone with the power to do what you are suggesting would be much more powerful than either of you. What would such a person possibly have to gain from confusing you? Even if Lord Voldemort had become that powerful, he would simply kill you rather than confuse you."

_Not true_, thought Harry. _If the defence professor's theory were correct, then Voldemort couldn't possibly kill me on pain of paradox. Not until I have sent my dark side back in time, at which point I would be fair game. If Voldemort could confuse me into doing it sooner rather than later, then there could be gains. Or there could be some other cosmic player who was just trying to get the threat of paradox past._

"Anyway," continued the headmaster, "We still haven't got to the question of how you did it, let alone why."

"I can do it again, if you like."

"I would greatly appreciate that."

Harry cast _finite incantatem_ on the gemstone that he had brought into the room. Harry's lattice vanished, along with the sense of power. Professor McGonagall's transformation remained – Harry's magic was not powerful enough to cancel hers. Harry visualized the lattice structure with the help of the diadem, and did the transformation. It took less time than previously – he must be getting practice.

Harry handed the gemstone back to the headmaster. It had the feeling of power back now.

"Merely that." The headmaster looked on with amazement. "How did you know that that would create the philosopher's stone?"

"I had absolutely no idea. I thought the philosopher's stone was just a myth. I was just trying a scientific experiment, making a cryptographic key. It just happened to result in this."

The headmaster looked at him intently. "Harry, I want to make sure that you understand me perfectly. I often appear to be flippant, but I want you to know that I am absolutely serious on this point. You must be careful doing _scientific experiments_. I have another prophecy that I suspect is about you: **_"HE IS COMING. THE ONE WHO WILL TEAR APART THE VERY STARS IN HEAVEN. HE IS COMING. HE IS THE END OF THE WORLD."_** That sounds like a reference to you. Who else is likely to end the world?"

"Possibly the hypothesised absurdly powerful deceiver," said Harry. "But I get your point. I will think about everything even more carefully." The simplest interpretation would be Harry neglecting to retrospectively create Lord Voldemort, creating a paradox, and wiping out a now impossible universe. An alternative interpretation would be Harry doing the ritual, which was actually a trap to make Lord Voldemort more powerful and destructive. Or maybe it was Harry doing the ritual, becoming a god, and then doing cosmic engineering. "Are prophecies always so frustratingly lacking in important specifics?"

"As far as I know, Harry, yes."

"I need to think. I need to just sit down for a few days and think without distractions."

"Always a wise plan. If I may suggest… could you please destroy your copy of the philosopher's stone? You can of course always recreate it at will. It would be a disaster if Lord Voldemort got his hands on it. He would be restored to his full powers."

"Wait! Does that mean it could resurrect Hermione?" _If you Dumbledore had the philosopher's stone in your office and denied her life out of some insane fatalistic religious convictions, then I immediately assign to you the title of public enemy number one._

"No, Harry. It can do amazing healing, but it cannot bring back the dead. Lord Voldemort is a special case as he is not fully dead, having created several Horcruxes. Whatever you do, do not let the stone touch the diadem."

"No, no," said Harry weakly. He cast _finite incantatem_ again on his copy of the stone, and then pocketed the inert gem. He briefly worried about the knowledge of the pattern being stored in the diadem, but clearly some embodiment was required too.

"And I shall now have to find a new hiding place for the one I am protecting."

"Out of Hogwarts! Get that dark lord magnet away from the kids. Better still, destroy it. I can easily make another one if needed."

"I see your point, Harry. But I am somewhat restricted in my ability. I will certainly bring it up again immediately with Nicolas Flamel. He was very specific about it. He and I need to have a long talk. At the risk of saying something utterly obvious, you should keep this ability of yours very secret. You are enough of a target as it is."

"Of course." Harry paused, wondering if a tirade against Nicolas Flamel would be helpful. Probably not. Instead he asked in a small voice, "One other thing. I would like to become a better person. I am seriously worried about what I might accidentally do through being self-absorbed. Do you know any magic that would make me… nicer? Less evil?"

"All such magic would be unforgivable."

"I don't mind. I am scared of myself."

"Your heart is in the right place, Harry. Trust it. Your brain… well, I cannot deny that it is very powerful. But sometimes you need to think things through just a little longer. Make a habit of it, and it will become second nature."

Harry was hoping for a more specific answer, or at least an opening to try on the sorting hat. He considered using the time turner and invisibility cloak to get it, but that may fail if Dumbledore spent too long in his office. Besides, there were probably wards all around, and the portraits were watching. The hat would tell him little he didn't know, anyway.

Harry and Professor McGonagall said their goodbyes. Out in the corridor, having noted that no one was near, she spoke to him.

"You didn't tell him why you made it. Do you not trust him to know?"

"It is not that I distrust him; rather I am not sure that I trust the portraits or the office's defences against scrying. If the true purpose got out, I would be even more of a dark lord magnet. It seemed that the fewer people knew about it, the safer I was. "

Professor McGonagall nodded.


	5. I want to become a better person

The defence professor was still alive. But he moved little other than his eyes when Harry entered.

"I have another present for you."

"I did forbid you from getting another unicorn."

Harry handed him a glass. "This was water. I dipped the philosopher's stone in it. Hopefully it is Elixir of life now."

The defence professor jumped up at that. "You never cease to surprise me, Harry." The side of his lips twitch up. "Few people ever start. How did you obtain that?"

The professor hadn't actually drunk it. Harry understood. It would be an easy way for Harry to assassinate him, should he wish to do so. Harry had given every indication of good will, but in light of the last conversation, there was indeed a small chance that Harry might decide not to go through with the ritual, and to kill the only other person who might make a fuss about it. It would be a surprising thing for Harry to do, but maybe the professor's comment had multiple levels.

"Dumbledore handed me the stone briefly. I made the elixir without him observing." Both true, even if the stone that Harry had used was not the one Dumbledore had handed him.

"I told you not to ask Dumbledore about it!" The professor's voice was sharp.

"He brought it up. I had absolutely no idea that he was going to. It surprised me."

"Why did he do such a thing?"

"My best guess is that he wanted to reassure himself it was still where he had hidden it."

"Let me guess. It was not in the chamber that he got all of us to make a guard for."

"I feel that it would be a breach of confidence for me to answer that question."

The defence professor smiled, and drank the aromatic elixir. There was no immediate reaction, but ten seconds later he was sitting more erect. Harry exhaled in relief.

"Thank you, Harry. This will sustain me at least as long as a unicorn would have, with fewer side effects for me and many fewer for the unicorn."

"I expect I will be able to obtain another one when it is needed."

The defence professor looked up at him sharply. "How on earth can you anticipate that?" Harry shrugged. He couldn't think of any honest but safe answer.

"I will stop looking a gift unicorn in the mouth. Speaking of gifts, how is the diadem suiting you?"

Harry grinned. He was wearing it at that point, of course. "I never want to take it off. Thought is life to me. I am pretty sure I would not have obtained the elixir so easily without it."

The defence professor smiled somewhat bitterly. "Giving that to you was motivated primarily by a rare instance of altruism on my part. It seems somehow sad for it to have benefited me directly so quickly."

"Altruism is a result of evolution. It has benefits for the individual in that a society with somewhat altruistic members is more likely to flourish than one without, and this is good for everyone including the altruistic. Alternatively, powering up your friends is often productive."

"So. Are you altruistic enough to sacrifice the worst part of your nature?"

"I have decided to do so." Harry had thought long and hard about this. It did seem the right thing to do, and if it backfired by creating a super Voldemort, well, he would be faced by a Harry Potter able to cast _Becomus Goddus_. "I would like to do so as soon as possible, while I have the courage and you have your health." _And while it is fresh in your mind that my continued existence as a free thinking individual is necessary for your supply of elixir of life_. "What do I need to do?"

"I am not entirely sure. There are many things that are clear. It will be powered by your sacrifice. But what is not entirely clear to me is how to define what part of your personality is the part to be excised. You probably have a better idea than me. If you can define it, I can incorporate it into a ritual. I think it would be better discussed in privacy."

The professor changed into a snake, and hissed at Harry. "I, like you, am a man of multiple parts. The body you see stumbled onto and triggered a Horcrux of Lord Voldemort. I am part him. I am part David Monroe, who for a long time led the fight against Voldemort until Dumbledore stepped up. The personality of my original body has significantly softened Lord Voldemort's personality, as with you. David Monroe was very strong, and fought the good fight, as they say. But he was not what most people would call _nice_, and the lack of gratitude – or even cooperation - from the people he protected instilled a level of contempt for humanity. He vowed never to overestimate the decency of humanity again. Those two personalities are not that different, and I could not separate them easily. Nor would I want to – they are both part of me now. While I am somewhat ruthless, I feel no desire to hurt for the sake of doing so. That is really what made Lord Voldemort a monster. The David Monroe personality is the strongest in me."

It was a startling admission, but not so surprising. Harry had done his homework and knew much about David Monroe, and had suspected a Lord Voldemort link with the defence professor for a long time. Harry probably wouldn't have noticed the slightly strange phrasing of one portion if it were not for the diadem. The professor had implied that it was David Monroe who had triggered the Horcrux and merged with Lord Voldemort. But if so he probably would have used slightly different phrasing. An entirely plausible alternative interpretation was that David Monroe and Lord Voldemort had always been the same person, and it was some hapless third party who triggered the Horcrux and acquired the two of them. That could explain the long inconclusive war – it would be easy to drag a war on if you led both sides. So maybe the whole war, the whole character of Lord Voldemort was a sham.

If it was a sham, why? Presumably to get David Monroe political power. He would acquire a reputation as a dangerous, scary, hero. Then he would take control of magical Britain – people would be motivated by _both_ fear of him _and_ gratitude towards him. A heady combination that. Then he would finally destroy the stalking horse Lord Voldemort, and have his position cemented. He wouldn't even have to worry about the heroes coming to get him, or the list of things an evil overlord should never do.

But he had made a big mistake. He had overestimated the finer feelings of humanity. It probably never occurred to him that this was possible. That had to hurt. Maybe it explained the professor's insistent lack of belief in true altruism. Then of course he made a bigger mistake, attacking Harry and incurring the wrath of the universe's paradox avoidance mechanism.

The speculation was important, but the most time sensitive thing was to verify that it did not invalidate Harry's decision to go through with the ritual. What ulterior motives could the defence professor have if this were the case? Clearly, the ostensible goal of maintaining the existence of the universe as he knew it was still valid.

A nasty thought crossed Harry's mind. If David Monroe had not received enough gratitude, then maybe Harry Potter had. Maybe the defence professor was intending to empty Harry's mind with the ritual, and then take over the empty husk of his body without the dilution of self that would come from a Horcrux merge. Particularly since the effect that stopped their magic working on each other would presumably vanish after the ritual. Harry's heart dropped. Right from the start, the defence professor had taught Harry how to lose. How to stop fighting. Maybe that had a secondary goal of making it easier for his mind to be taken over.

So. That just meant that he needed to make sure that the ritual did not wipe out all of his personality, and that the very first thing he did after the ritual was cast the Becomus Goddus spell. After all, everything would be easy after that was done successfully.

Furthermore, the upside for humanity of the Becomus Goddus spell working was so high that the risk of the ritual backfiring was worth taking. Harry had to admit it was not just altruism talking – it was the easiest way he could see to resurrect Hermione. And that was not selfless, that was personal.

"When I get angry," said Harry at last, "I get a feeling of cold clarity. Then I can think about the best solution to my problems, without any regard for morality. It feels almost like a different personality. That is the easiest to recognise portion of my personality as the Voldemort fragment. But it may be more than that. Part of me says that whatever I guess will turn out to be correct since by time loop I must have already done it; another part of me is rather scared by such logic and demands that I treat the subject with all the care I can."

"Agreed. Care is good."

Harry considered the fixed point of nobility he had discussed with professor McGonagall. That seemed likely to pass the "User too evil" test of his stone, but it may very well not pass the "user still conscious" test. There was a weaker version that would probably pass both tests. "I have a suggestion. It may not be possible to express in a ritual, but may be worth suggesting as a basis."

"Go ahead. If you can express in in English, there is a good chance I can express it in a ritual."

"Call my personality H. Let H' be the personality left when I have lost the aspects of my personality that I consider evil. Let H'' be H', less the additional pieces of personality that H' considers evil. Continue this process until you stop removing pieces. By definition, the resulting person cannot consider himself evil." _And therefore I should pass my own test_.

The snake that was the professor considered that for a while.

"You make it too complex, out of habit of muggle mathematical language. I suggest that the following is equivalent: `I continually sacrifice those parts of myself I consider evil, until there is no part left of me that I consider evil.'"

Harry nodded.

"In that case, student, you just say that phrase at the same time I do my portion of the ritual. Could you repeat it for me please to make sure you have it."

With the help of the diadem perfect memory was effortless. Nevertheless, Harry obediently repeated "I continually sacrifice those parts of myself I consider evil, until there is no part left of me that I consider evil."

The snake converted back to the tired figure of the defence professor. "Regardless of what happens, Harry, I want you to know that you have been the best company I have had in my life. Are you ready?"

"I am." He didn't feel ready. He felt as if he was about to make the biggest mistake of his life.

"Then start."

Harry started. The professor spoke rapidly in a language Harry did not recognise, all the while waving his wand. Thrice the professor grabbed items off his desk, and threw them in the air where they vanished in green mist. Harry began to feel faint, feel a pulling in his head. A woman, possibly Bellatrix Back, appeared, holding the defence professor's hand. Harry finished his sentence. Then there was blackness.


	6. Lack of experience

**Elsewhen**

Tom mastered his instinctive nausea as Myrtle fell down, slain by the Basilisk. Slain by the will, if not the hand of Tom Riddle, precocious and very ambitious student. He hadn't planned this sufficiently in advance. He could think of two rituals to power with her death. Creating a Horcrux, say into his diary was the obvious choice – first get immortality, then he will have time for everything else. But. But it was important to get the Horcruxes perfect, and he wasn't absolutely sure about it. He needed more knowledge, more intelligence first.

So it would have to be the other way round. Knowledge and intelligence first. He used her death to summon the largest nearby intelligence of the dead.

It came from somewhere unrecognisable.

It was _strong_.

**Present**

The defence professor felt the power flowing into him. The missing part of Tom Riddle that had been lodged in Harry bifurcated. The smaller part went back in time; the larger part flowed directly to the defence professor in the present. He felt _strong_. The fragment had flourished in Harry's fertile mind and was far greater than the amount lost when his Lord Voldemort puppet burned in the Harry Potter fiasco.

The small amount of decency that the venal Quirinus Quirrell had provided was swamped by the strength of Tom's reascending personality. Even thirty seconds ago, the defence professor had been unsure if he could fully go through with his plan. Those doubts were gone. His fondness for the child at his feet was swamped by the anger at people who had not looked up to David Monroe. People who thought more highly of a baby saved by a time loop than of a grown man actively putting himself in harm's way again and again to protect the ungrateful wretches.

Harry fell forward, unconscious. The professor cast _enervate_ at him. It worked. Harry crawled to his knees. The professor had been fairly sure that the time loop would be fully resolved now, and that the immunity Harry had possessed to his magic would be gone, but it was a relief to have it actually shown correct. He smiled. In minutes he would be in Harry's body - the body of the popular hero that the Monroe puppet never was.

Just in case, Tom, as he now thought of himself, cast a powerful _finite incantatem_ at Harry. There was a tiny chance the boy would retain enough sentience to repeat a previous trick. The gem on Harry's ring rapidly grew into the large rock it was naturally. As he suspected, but had not been sure of, the ring itself turned into Hermione's half eaten corpse.

There was a bang and a flash, and Dumbledore was there, McGonagall by his side. The wards had been triggered. Tom smiled. All part of the plan. There would be a big battle; Dumbledore would kill the new Lord Voldemort and Bellatrix Black, cementing his reputation and power. McGonagall would be witness. Unknown to her, the real Tom Riddle would now be inside Harry Potter. Then Harry and Dumbledore together would rule Britain with strength.

Tom hesitated a fraction of a second. It had taken him a long time to persuade Dumbledore that this was the best possible outcome for the world. Ironically it had been easier to persuade Harry of the necessity for his sacrifice to stabilize the time loop than it was for Dumbledore. Hermione's death had almost put Dumbledore off script too.

It had taken letting Dumbledore freely root through his brain to persuade him that it was all true, and that his intention was to honour the agreement.

But Tom was no longer as conscience bound as the entity Dumbledore had read the brain of. The thing was, Tom didn't absolutely need Dumbledore. And he would be much safer in a Dumbledore free world. While Dumbledore and McGonagall were raising their wands, Tom cast _Avada Kedavra_ at Dumbledore in half the time that Lord Voldemort had ever publically demonstrated.

Harry's hand had reappeared, having presumably been under his invisibility cloak. It was holding an artefact of vast power. The philosopher's stone, it must be. Not a threat. His lips were whispering something. Not important.

McGonagall though had reacted faster than Tom had anticipated. Half a second was all that was needed for the transfiguration expert to convert Tom's wand into a pumpkin. Tom didn't waste time castigating himself. He physically punched her, hard, and she went down, winded. Tom fell with her, pinning her wand arm and preventing a follow through. On the ground near Harry he grabbed the wand, twin to his original, from Harry's unresisting hand. Harry was saying something like "Becomus" Tom turned back to the somewhat stunned Minerva. He needed her to survive, to be a witness to Lord Voldemort losing to Harry Potter again. But he couldn't have her interfering. With Harry's wand he cast a binding spell on her from point blank range.

Tom spun back around to Harry, who had just finished saying something like "Goddus". The philosopher's stone beeped and announced "Insufficient experience points error." Harry screamed "No!" in horror.

The philosopher's stone wasn't totally finished. A beam shone out to each of the people – or bodies – around it. Tom tried to defend, but it went through all his shields. Then he stopped worrying. He felt great! All the accumulated damage from a lifetime of black rituals had vanished.

Study later. Fight now. He cast an absurdly strong _Imperius_ on Harry before further things went wrong, and looked around. Bellatrix was standing tall. He smiled to see it. McGonagall had recovered from her binding spell and had started some invocation. No problem. He bound her again, faster than he could have even five seconds ago.

To his bigger surprise, Dumbledore and Hermione were also recovering from their death. Dumbledore was not fast enough, though. Before he had picked up his wand, Tom killed him again.

Tom turned to Harry. Hermione living again was a great opportunity for Tom to crush what remained of Harry's mind. It would make taking over his body easy. "Kill Hermione," he ordered.


	7. I am not the heir to Ravenclaw

Hermione was sure she was about to die. Not that she had been thinking clearly, with the rapid blood loss. She had not expected an afterlife, and certainly not in an ongoing battle in the defence professor's office. She recognised the defence professor, saw him kill the headmaster with a green spell, presumably _Avada Kedavra_, although it was spoken too quickly to tell. A wave of anger passed over her. She had never liked or trusted him, and was horrified at his campaign to poison Harry's character. He probably set the troll on her. He had arranged for her to be _eaten_. For the first time in her life she truly hated someone.

Reflect on motivations later; concentrate on the present danger first. Her body looked whole. She felt better than ever. Harry must have pulled some spectacular rabbit out of his hat, and, true to form, got himself in major trouble as a result. Her wand was absent, but Dumbledore's had fallen near her. She picked it up, her fine memory recognising it as the elder wand. It would do.

"Kill Hermione," said the defence professor. It looked as if he was speaking to Harry.

Harry was looking at her in horror. _Imperius_, she guessed. If she could kill the defence professor she should be fine. It appeared he was not feeling threatened by her; she might be able to surprise him with her unaccustomed vim. She cast a blindingly fast _stupify_ charm at the defence professor. A thick red bolt left her wand, and, too fast to see clearly, swerved around the defence professor. That approach wasn't going to work.

Harry was losing the battle to resist the curse. He looked down at his pouch. That could not be good. She recognised the diadem on his head. She had seen paintings of Rowena wearing it. The colour was red rather than blue, but that was probably insignificant. _Accio diadem_, she cast. It flew rapidly off his head into her hand. Harry stumbled, and looked confused for a moment.

The diadem blazed with blue light when Hermione put it on her head. It was _wonderful_. It meshed so perfectly with her mind that she was convinced it could have been designed specifically for her. She felt like a Goddess. Everything was so obvious. The crease in Harry's robes above his chest, for instance. That was most likely from a Time Turner. She had thought them myth, but it would explain so many of his otherwise impossible feats. The _cheater_!

It didn't matter. Now she had the diadem, she could cheat herself. She could outthink him. Her magic was more powerful than his; he didn't have a chance in a fair fight, even if he had a wand. But it wasn't remotely fair. While he was handicapped by his attempts to fight the imperious, she had a far bigger handicap. Killing Harry was not something she would let herself do. And even if she did, they would both be at the mercy of the defence professor, who had clearly just effortlessly killed Dumbledore.

She had one advantage. The defence professor underestimated her. He was sure she would not kill Harry, and he had made no move to help Harry in the fight against her. She hated when adults underestimated her. Without letting her expression change, she fed the rage.

Harry had regained his composure from losing the diadem. "I am sorry, Hermione. There is no good solution here. We have lost. I made a gamble, and it failed. There is no way I am going to leave this in any way recognisable as myself. Neither are you. The most likely outcome is that you spend the rest of your life under Imperius, forced to do terrible things, wishing you were dead. Just like me now. Your best option is to kill yourself pre-emptively. I assure you from personal experience that there is nothing you want less than to end up like me now."

Possibly true. She hated what the monster had done to Harry, to Dumbledore, to herself. She hated like she had never done in her life. Then she spun the elder wand around at the defence professor again. _Avada Kedavra_. Unthinkable. Unforgivable. _Unblockable_.

Nothing happened.

The defence professor smirked in contempt. She had predicted that, but even so, it made her anger blaze for a moment. _Avada Kedavra_.

This time, a bolt of green energy streamed out of her wand. Try deflecting that, condescending murderous dark lord possessing my friend, she thought in triumph

The defence professor had not quite as underestimated her as she had hoped. He was ready on the balls of his feet, and he simply dodged. "Bravo, Miss Granger," he said, bowing without taking his eyes off her.

She turned back to Harry, who had changed tactics. He had just taken something out of his moleskin pouch, and was now throwing it at her. Danger. Several seconds ago, her time, he had blown up the troll's head with a similar move. She deflected the unknown objects with a simple shield charm. His other hand held some stone. Her mind split into thousands of threads, analysing it. One noticed that it matched vague references to the shape of the philosopher's stone. That must be how she was alive. Harry changed tactics and started putting his invisibility cloak on. Danger. She would be vulnerable to physical attack if he were invisible. She needed to disable him before that happened.

She couldn't break the defence professor's Imperius. Her magic was not nearly strong enough, even with the elder wand. She couldn't even claim to be its true owner. The only other way she knew to lift an Imperius curse was to arrange for the caster to not be alive. Her mind split into tens of thousands of threads as she thought through the space of possibilities.

Got it.

She imagined the tiny piece of glass at the narrowest point on the time turner, pointed the elder wand at the lump on Harry's chest, and cast _Accio fragment_.

There was the faint sound of breaking glass.

Then _something_ happened. There was falling involved.

**Elsewhen**

Harry didn't pick himself up immediately. That would be threatening. He let go of the cloak and the philosopher's stone, and put his empty hands up in the air. He felt badly bruised and totally ridiculous, sprawled on the ground. That was fine. Ridiculous was an improvement on ten seconds ago, subjective. He said "I am free of Imperius, Hermione. Thank you, and magnificently done."

She picked herself up gingerly and looked around, maintaining the wand pointed directly at Harry. The two of them were alone in a grassy meadow where Hogwarts would one day be built. The surrounding countryside looked right, apart from the lack of a school. Her plan had worked. The defence professor was not alive _now_. She lowered her wand.

"I guess we're on our own now," she said. "Two small children with nothing to protect us other than our wits, the elder wand, the philosopher's stone, the true cloak of invisibility, and Rowena Ravenclaw's diadem."

"Not to mention the contents of my moleskin pouch," said Harry, picking himself up. "After your death, I loaded it up with quite a collection of muggle artefacts. I don't think we should have many problems looking after ourselves, assuming we are still in the era of homo sapiens."

"Oh yes. We are about three thousand years back. Certainly no more than five thousand years. Geology." She smirked, waving an arm at the scenery. She was keeping the diadem. It would make Harry's company less aggravating.

"I expect I could learn a new language in weeks, with the help of the diadem." Harry tentatively raised a hand a fraction, the merest hint of a request. Hermione ignored it.

"I think I could learn one in days," she replied. "We should probably use assumed names. We want to be careful not to cause paradoxes."

Harry shivered. "Yes," he said. "I wonder how many of the people we have heard of we will be. It is a good thing I have several history books in my pouch. Fancy yourself as Rowena? We will live for a long time, with the philosopher's stone. Time to gain experience."


	8. The end of the world

Nicolas Flamel looked at his watch. If his memory was correct, the battle should be over by now. At last the heartbreaking millennia of limited intervention were over; the children were safely gone and he did not have to be paranoid about paradoxes, at least from them. If all went well there would be just one more big time trip, creating magic. He realised he was procrastinating, and waved a very old wand over the watch; it turned into a blood red gem with an array of imperfections in the middle. "Becomus Goddus," he whispered.

The stone immediately beeped. "Invalid hairstyle error," it announced in the voice of his childhood.

"What!" exclaimed the wise old mage, losing his equanimity for the first time in centuries.

Hermione, as he still thought of her, laughed. "You must have forgotten how obnoxious you can be." There was no barb in her voice… she knew he did nothing but mean well. She waved her wand at her husband, and his hairstyle changed to that he wore when he first met her on the train to Hogwarts.

"Just kidding!" Added the stone.

The sun dimmed for seven minutes, and then the new deity looked up and smiled. Destroying death would take huge magical resources, but enough would be freed by a new interdict stopping time travel. This would have the side benefit of preventing the universe from vanishing in a paradox.

* * *

><p>The centaurs were the first to notice the change. They looked up at points of light in the sky with horror. There was no information travelling back in time through them; they were no longer stars as far as prophecy was concerned. They were merely far off masses of fusing plasma.<p>

Seven human seers suddenly broke off in the middle of their prophecy of an imminent end of the world. They were left with a puzzled feeling that their world _had_ actually ended, although it certainly seemed to be still present when they looked with their prosaic eyes.

In the Hall of Prophecy, shelves full of glowing orbs darkened.

But there was light too. In Azkaban it was the brightest, and when it dimmed, there were no dementors left.

* * *

><p>Authors note:<p>

Technically the title should be "Harry Potter and the Cryptographic Quality Key", as there is no actual cryptography involved; just a key that has many of the properties of many cryptographic keys. But that sounds clumsy, as does "Harry Potter and the eight thousand random bits".

If you like this I have some original fiction at fictionpress, a hard Sci-Fi novel called "Artie", with mostly rational characters.


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